1. Field of the Invention
This invention relates to a process for the production of elemental liquid sulfur from CO.sub.2 -rich gaseous streams containing H.sub.2 S, such as refinery gases, natural and vent gases from natural gas processing plants, and tail gas from Claus type sulfur recovery units.
2. Description of the Background Information
One of the processes extensively used to recover elemental sulfur from H.sub.2 S-bearing gas streams is the Claus process. A typical Claus process involves reacting one third of the source H.sub.2 S with air in a combustion chamber to form SO.sub.2 : EQU 2H.sub.2 S+30.sub.2 .revreaction.2H.sub.2 O+2SO.sub.2 EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O
The remainder of the H.sub.2 S is combined with the SO.sub.2 formed to yield elemental sulfur vapor. The effluent from the combustion chamber is cooled, and the free sulfur is recovered as a liquid. The remaining unconverted gases are usually passed through two or three catalytic sulfur reactors to further convert the unreacted hydrogen sulfide and sulfur dioxide to free sulfur. The desulfurization afforded by the Claus process is about 90% to 97% effective in producing elemental sulfur from H.sub.2 S. The tail gas from a Claus unit typically contains an undesirable COS contaminant produced by the following side reactions: EQU CO.sub.2 +H.sub.2 S.fwdarw.COS+H.sub.2 O EQU CO+1/2S.sub.2 .fwdarw.COS EQU CH.sub.4 +SO.sub.2 .fwdarw.COS+H.sub.2 O+H.sub.2
Another well-known process for converting hydrogen sulfide to elemental sulfur is the Stretford process. In the Stretford process, a hydrogen sulfide-containing feed gas is treated by contacting the gas with an aqueous alkaline solution to produce an effluent gas of reduced sulfur content and to yield elemental sulfur; the solution is thereafter regenerated by contact with an oxygen-containing gas; the elemental sulfur is recovered from said solution; and the regenerated solution is recycled to the gas-contacting step.
U.S. Pat. No. 3,034,865 to P. Urban also teaches a process for converting hydrogen sulfide to elemental sulfur which comprises forming a reaction mixture of hydrogen sulfide and a basic nitrogen-compound solution and reacting the mixture with oxygen in the presence of a metal phthalocyanine catalyst.
U.S. Pat. No. 4,036,942 to Sibeud et al. discloses a process for the removal of hydrogen sulfide and alkyl mercaptans contained in liquid and gaseous streams. The subject process is a continuous and cyclic process which involves converting the hydrogen sulfide and alkyl mercaptans by catalytic oxidation using an aqueous metal chelate solution as the catalyst to form elemental sulfur and dialkyl disulfides. The metal chelate catalyst solution is recovered and recycled after separation of sulfur and dialkyl disulfides. Specifically the fluid stream containing hydrogen sulfide and alkyl mercaptans is contacted with free oxygen gas and the aqueous chelate catalyst solution in the presence of an amine. An inorganic alkali is added in an amount to adjust the pH from 6.8 to 10.
U.S. Pat. No. 3,956,473 to Mimoun et al. teaches the catalytic oxidation of hydrogen sulfide to sulfur in homogeneous liquid phase, by means of molecular oxygen, in an organic solvent whose water content is lower than 60% by volume. The liquid phase comprises a solvent selected from amides, phosphoramides, sulfoxides, nitriles and ethers. The oxidation is conducted in the presence of at least one catalyst selected from the two following categories:
a. a chelate of a transition metal selected from the group of metals consisting of iron, manganese, copper and cobalt, the chelate being selected from the group consisting of dialkyldithiocarbamates, alkylxanthates, dithiolates, 8-hydroxyquinolinates and salicylaldimines,
b. a salt of such transition metal of the formula MX.sub.m in which M is the transition metal, X is an anion selected from the group consisting of Cl, Br, CIO.sub.4.sup.-, NO.sub.3.sup.- and SCN.sup.- and m is 2 or 3.
U.S. Pat. No. 3,423,180 to J. Hoekstra pertains to the oxidation of a soluble sulfide compound. The process comprises the steps of introducing an aqueous solution of the sulfide compound together with oxygen into a reaction zone containing a solid oxidizing catalyst.
U.S. Pat. No. 3,205,164 to K. M. Brown teaches the conversion of H.sub.2 S to form thiosulfate. The H.sub.2 S contained in a hydrocarbon fraction is contacted with oxygen and a metal phthalocyanine catalyst in an aqueous solution, preferably an alkanolamine solution.
U.S. Pat. No. 3,039,855 to P. Urban teaches a process for oxidizing hydrogen sulfide and mercaptans to an oxidation product selected from the group consisting of elemental sulfur and disulfides which comprises reacting the sulfur compound with an oxidizing agent in the presence of a phthalocyanine catalyst at a pH of not greater than 7. Patentee carries out the oxidation of gaseous sulfur compound in several ways. When the sulfur compound is recovered as a gas, it may be passed, together with air, through a pool of a solution of the phthalocyanine catalyst at a pH of not greater than 7 or the gas may be passed upwardly through a descending solution of the phthalocyanine catalyst in a neutral or acidic medium. The gaseous sulfur compound may alternatively be passed into contact with a solid bed and the sulfur compound adsorbed on the solid bed. The solid bed may contain an oxidation catalyst or comprise a suitable adsorbent not containing the catalyst. When the adsorbent contains the catalyst, the adsorption of the sulfur compound is effected first and then air or other oxygen-containing gas introduced to effect the desired oxidation, or air is introduced along with the sulfur-containing gas and oxidation effected in one step.
U.S. Pat. No. 4,196,183 to Li discloses a process for removing H.sub.2 S from geothermal steam by adding free oxygen to the steam and contacting the mixture with an activated carbon catalyst at a temperature above the saturation temperature of steam. In Li's process the H.sub.2 S is oxidized to free sulfur which is absorbed on the activated carbon and which periodically must be removed therefrom. Li explains in his text that the superheating is necessary to prevent binding of active catalysts sites by moisture.
U.S. Pat. No. 3,972,988 to Urban discloses a two-stage process for obtaining free sulfur from an aqueous sulfide solution. In the first stage, the sulfide is converted to a polysulfide which is then mixed with steam and decomposed to form a mixture which might be considered analogous to geothermal steam in that it comprises vapor containing H.sub.2 O, H.sub.2 S and NH.sub.3. This vaporous mixture is then mixed with air and contacted with a metal phthalocyanine catalyst in a second stage at a pressure not greater than 20 psig to produce sulfur (which maximum pressure is stated to be the essence of the invention), and a solution of a relatively minor amount of thiosulfate compound which can be disposed of or recycled to maintain a thoroughly wetted support bed. The stated advantage to the low pressure of the second stage is that the oxidation is performed in a vaporous phase, whereas in the prior art the oxidation is performed in a liquid phase.
Finally, U.S. Pat. No. 4,358,427 to P. Urban discloses a process which is used to remove H.sub.2 S from geothermal steam which geothermal steam contains less than 1,000 ppm by volume H.sub.2 S.